L 



[526] 



P A 



there occurs a bed of dark blue 

 clay, called Oxford, or clunch, clay, 

 the thickness of which has been 

 stated to be 200 feet. Between 

 the middle and upper also, there is 

 found a thick bed of clay, called 

 Kimmeridge clay, of a thickness 

 exceeding, in some parts, 100 feet. 

 The uppermost members of the 

 oolite group are the Portland beds, 

 lying immediately under the Pur- 

 beck beds. 



Oolite has been also called roe- 

 stone, from a supposition of the 

 older geologist that the globules 

 contained in it were the petrified 

 roes of fishes. In the lithographic 

 limestone of Solenbofen, belonging 

 to one of the upper members of the 

 oolite, a great variety of organic 

 remains is found ; and in the 

 museum of Count Munster, there 

 are not fewer than seven species of 

 flying lizards, six saurians, three 

 tortoises, sixty species of fishes, 

 forty-six species of Crustacea, and 

 twenty-six species of insects, taken 

 from that deposit. 



The oolitic tracts of England 

 present a broad band of dry lime- 

 stone surface, rising westward to 

 elevations of from 800 to 1,400 

 feet, with escarpments commanding 

 very extensive prospects over the 

 undulating plains of lias and red 

 marl. The whole tortuous line of 

 oolitic escarpment from the Humber 

 to the Avon, may be regarded as 

 the wasting effects of water on the 

 subjacent red marls and lias clays. 

 BakewelL De la Beche. Lyell. 

 Cleaveland. Mantell. Phillips. 

 Conybeare. 



OOLIT'IC. Composed of oolite ; re- 

 sembling oolite. The name of a 

 large group of strata commencing 

 with the Portland beds above, and 

 terminating in the inferior oolite 

 below. 



OOLI'TIC PEB'IOD. Called also the 

 Jurassic period. The Oolitic period , 

 i comprised in the secondary or 



Mesozoie epoch, and is placed 

 between the cretaceous and triassic 

 periods. The series of rocks de- 

 posited in the British isles during 

 the Oolitic period is so complete 

 both petrologically and palaeonto- 

 logically, that they serve for a 

 type for those known all over the 

 world. 



OOLITI'FEROTJS. Producing oolite, or 

 roe-stone. 



OPA'CITT. (opacitas, darkness, Lat. 

 opacite, Fr. opacitd, It.) Opaque- 

 ness; darkness. The quality of 

 opacity is not a contrary or antago- 

 nist quality to that of transparency, 

 but only its extreme lowest degree. 



OPAL. The quartz resinite of "Werner ; 

 untheilbarer quartz of Mohs; a 

 sub-species of indivisible quartz. 

 Of this there are many varieties, 

 the principal of which are, 

 1. The precious opal, a milk-white 

 variety, with a beautiful play of 

 various rich colours. The play of 

 colours is supposed to be caused by 

 numerous minute rents that tra- 

 verse this mineral ; thin layers 

 being contained in them, which 

 have the property of reflecting the 

 prismatic colours. The precious 

 opal is sometimes imperfectly imi- 

 tated by artificial glasses ; and 

 substances which resemble the opal 

 in its play of colours are said to 

 opalesce. 2. Fire -opal ; a trans- 

 parent variety, brought from Mexi- 

 co, with a carmine-red and apple- 

 green iridescence of great beauty. 

 Found also in Hungary and Corn- 

 wall. 3. Common opal; a variety 

 differing but little from the precious 

 opal in many of its characters, but 

 not presenting that effulgence, or 

 play of colours, by which the 

 precious opal is distinguished. Its 

 colour is white, shaded with grey, 

 green, or yellow, sometimes milk- 

 white. WTien viewed by trans- 

 mitted light, the milk-white and 

 greenish varieties often change 

 their colours. These varieties con- 



